Continuing improvements in digital communication technology have made it increasingly attractive for the RBOC's to offer digital transmission service to their customers as a cost effective and accurate method of sending and receiving data from facilities, such as businesses, homes, hotels, dormitories, etc. One such service is the digital data service (DDS), which uses time division multiplexers (TDM) for multiplexing high speed serial data into a series of time increments, each providing a data stream having a transmission rate of, for example, up to 64,000 bits per second.
The DDS subscriber is connected to the network through an office channel unit (OCU), or office channel unit data port (OCU DP) using a four wire metallic circuit consisting of two conductor pairs, each pair being associated with a respective direction of transmission, hereinafter referred to as `downstream`--for transmissions from the office channel unit to the subscriber central office, and `upstream`--for transmissions from the subscriber to the office channel unit. (For a detailed description of office channel units (OCUs) and a DDS in particular, attention may be directed to literature published on the subject, such as the November 1982 issue of the Bell Technical Journal.)
A problem in providing digital data services is the need to install in the link downstream of the channel unit special equipment that is capable of interpreting digitally formatted (test) command signals (e.g. digital loopback command signals) in order allow the service provider (RBOC) to properly test the service to the circuit remotely, without subscriber involvement. Such test interface devices, commonly known as digital data station terminations, or DDSTs, are located at the subscriber end of the loop. The DDST contains active electronic circuits which receive signals to be exchanged between the central office and the customer's equipment. Upon detecting and interpreting digital command signals, the DDST is operative to cause the signals to be returned, or `looped back` to the sender for test purposes. Such loopback capability allows the central office to send known digital signal patterns, and receive these same patterns via the loopback or return path, so that the accuracy of the transmission service may be determined. The test carried out involves bit error rate testing (BERT) and provides the RBOC with performance information to insure that the service to the customer is satisfactory.
Because the physical length of a subscriber loop may be quite substantial (e.g. up to 60,000 feet), the signals conveyed over the loop are subject to a significant amount of degradation, resulting in distortion and/or loss of information. As a consequence, to successfully carry out loopback testing for such long line links, the subscriber loop must be modified to accommodate the insertion of additional complex circuitry which performs amplification and distortion-removal prior to retransmission of the digitally encoded test patterns back to the central office.
In order to initiate the testing of a digital services link, a test system (such as a "SARTS" manufactured by AT&T), that is remote from the central office, transmits prescribed digital command signals associated with a loopback function. At the central office, these digital command signals are passed on by the office channel unit to the individual downstream circuits via the telephone links in which the circuits are installed. Normally, a device (e.g. an OCU DP) receiving a loopback command from the test facility causes the current within the four wire loop to be reversed, with the current reversal being sensed by various downstream communication devices.
For example, when a loopback command code pattern is received by an office channel unit from the test facility, the OCU DP customarily operates a double pole-double throw relay, the contacts of which are coupled in circuit with the four wire telephone link, so as to cause reversal of the direction of flow of sealing current that is continuously supplied over the four wire link between the office channel unit and a (downstream) DDST. This reversal of sealing current, in turn, is sensed by the downstream DDST, which then disconnects the four wire loop from the customer equipment, and loops the digital command pattern, that has been forwarded down the link to the DDST, back up the upstream wire pair to the office channel unit for comparative analysis.
Thereafter, a second digital command code pattern is forwarded down the link to the DDST. The digital data processing circuitry within the DDST is configured to interpret this second command code and reconnect the four wire link to the customer's equipment, thereby passing the reversed current condition on to the customer. By convention, the customer's device interprets the reversed current condition as a loopback instruction, so that the data being conveyed over the downstream portion of the link is now looped back at the customer's equipment, whereby the functionality of the link may be tested all the way to the customer's equipment.
In summary, in a conventional digital services loopback testing scheme, the signal processing capability of the DDST must enable it to do the following. First, it must detect reversal of loop current and disconnect the customer from the link. Secondly, it must regenerate the digital data pattern, in order to overcome line losses and distortions, and then retransmit the received digital command signal back up the return or upstream portion of the link to the central office channel unit. The DDST itself then looks for a prescribed digital pattern from the channel unit and, upon detection of this digital pattern, the DDST reconnects the signal path back to the customer. The DDST also continues to monitor the direction of the loop current; when the direction of loop current flow returns to normal, the DDST returns to the normal data mode. The office channel unit must also detect in-band digital command signals from the test system and relay these digital command signals, as well as perform sealing current reversal as instructed by a command.